Laser Processing

We use ion implantation followed by pulsed-laser melting (II-PLM) to form many semiconductor alloys that are metastable or otherwise difficult to produce. Using this processing route, we have investigated the synthesis of Highly Mismatched Alloys (HMAs) such as GaAs_1-xN_x and ZnTe_1-xO_x as well as Diluted Magnetic Semiconductors (DMSs) such as Ga_1-xMn_xAs and Ga_1-xMn_xP. more...

Germanium grown on silicon has been a model system to study island self-assembly processes in semiconductors. Both materials are elemental, diamond-structure semiconductors and are fully miscible. Germanium growth on Si occurs via the Stranski-Krastanow (S-K) mode due to the 4% lattice mismatch between Ge and Si and the lower surface free energy of Ge. This growth mode is characterized by a transition from initially layer-by-layer growth to 3D island formation. Extensive investigations have been carried out on the structure and evolution of Ge islands on Si. Generally, germanium island ensembles are observed in a random distribution and are given names based upon their shape, e.g. huts (or pyramids), domes, and super-domes. A variety of growth techniques have been explored in the quest for spatial control of Ge islands, and to varying degrees, one- and two-dimensional ordering has been achieved. For example, selective epitaxial growth (SEG) techniques make use of Si mesas, dimples, or windows to direct nucleation. These routes for directed assembly often require substantial substrate-to-substrate patterning. It should also be noted that island growth continues as huts and domes despite patterning of the surface. more...

Diluted Magnetic Semiconductors (DMSs)are materials where magnetic ions are incorporated into a semiconductor matrix. Although these materials can exhibit a wide range of electronic and magnetic properties, most recent research has focused on attempting to synthesize DMSs that are ferromagnetic above room temperature and that can be incorporated into semiconductor devices. These efforts are in the hope of producing spintronic systems, where the manipulation of the spin degree of freedom is used as the basis for new devices. Although significant interest in DMS systems began with II-VI and IV-VI based systems in the 1970's, it is still in its exploratory stages with a large fraction of groups' efforts being devoted to the search for material systems with high Curie temperatures.

We have used demonstrated that the combination of ion implantation and pulsed-laser melting (II-PLM) can be used as a valuable tool for exploring these systems. To date, many of the most promising materials systems have only been synthesized by MBE, which is poorly suited for device fabrication. Our synthesis method, besides being much simpler than MBE, is also compatible with device fabrication techniques. more...

Highly mismatched alloys (HMAs) are compound semiconductor systems where the some of the anions are replaced by a much more electronegative element. Examples would be the addition of nitrogen to III-V systems like GaAs, InGaAs, or GaP and the addition of oxygen to II-VI systems like CdTe, ZnTe, or ZnSe. The highly electronegative ions tend to form spatially localized states which hybridize with the extended states of the host material. This mixing causes dramatic changes in the band structure - for example: large changes in the band gap of GaAs with only a few at% of N, a change in GaP from an indirect to a direct band gap with a few at% of N, and even the formation of a material having a distinct band located within the band gap leading to two direct band gaps at the G-point in ZnTe_1-xO_x. more...